1. Field of Invention
The present invention relates to improved inverter-type ballasts for fluorescent lamps.
2. Related Applications
The applicant has three related applications pending: Ser. No. 412,771 entitled Ballasts With Built-in Ground - Fault Protection; Ser. No. 481,714 entitled Inverter-Type Ballast With Ground-Fault Protection; and Ser. No. 500,841 entitled Series - Resonant Electronic Ballast Circuit.
3. Description of Prior Art
It is a basic safety requirement of fluorescent lighting fixtures that they be substantially free of serious electric shock hazard to persons involved with removing and/or replacing fluorescent lamps therein.
Electronic or inverter-type high - frequency ballast, especially as used with many of the recently developed high-efficacy fluorescent lamps, require particularly high voltages to achieve proper lamp starting and operation. As a result, the voltages provided at the lamp sockets are particularly high and therefore present special electric shock hazards.
To mitigate these special electric shock hazards, which is necessary to achieve listing by Underwriters Laboratories, manufactures of such inverter-type fluorescent lamp ballasts have resorted to the use of an isolation transformer at the ballast output, thereby to isolate the electrodes in the lamp sockets from ground; which, in turn, provides for the requisite mitigation of electric shock hazards.
Prior-art inverter-type ballasts wherein electric shock hazard mitigation is achieved by the use of an isolation transformer at the ballast output are described in U.S. Pat. No. 4,277,726 to Burke and U.S. Pat. No. 4,392,087 to Zansky.
Another approach at achieving acceptable mitigation of electric shock hazard is provided in U.S. Patent No. 4,370,600 to Zansky and involves an arrangement whereby the inverter automatically ceases to operate in case a lamp is disconnected from but one of its socket electrodes. However, Zansky's approach is problematic in that it does not permit the use of standard wiring connections to the fluorescent lamps in case of a mult-lamp ballast; which, in turn, makes it impossible to provide such a ballast as a direct retro-fit replacement for an ordinary multi-lamp ballast.